Design and Imaging of Nanoalloys by Advanced electron MIcroscopy for Catalysis – DINAMIC

Nanosciences and nanotechnologies have recently emerged and expanded considerably. Their success comes from an unprecedented ability to fabricate, manipulate and observe materials at the nanoscale. The interest for the “nanoworld” is due to the novel properties exhibited by nano-objects with respect to their bulk counterparts.
Catalysis is involved in most of industrial chemical processes for, e.g., refining, pollution control and synthesis of chemicals. Heterogeneous catalysis has always used nanoparticles in order to maximize the surface/volume ratio of active particles. Therefore, “particle size effect” is a well-known concept of catalysis. Moreover, combining metals within catalysts can lead to improved catalytic performances with respect to pure metals, e.g., increased selectivity or resistance to poisoning. The “alloying effect” is classically ascribed to either electronic structure or active site geometry. However, this phenomenon is poorly understood and controlled, due to the difficulty to elaborate homogeneous collections of multimetallic nanoparticles with imposed composition, and to the lack of structural characterization. Since the structure of multimetallic nanoparticles drives their catalytic properties, which are in turn affected by the reaction conditions, being able to rationally design a selective and stable catalyst requests the perfect knowledge of particle structure and the mechanisms of structural modification. This implies important efforts in both synthesis and characterization of “nanoalloys”.

The DINAMIC project deals with structural aspects of catalysis by nanoalloys.
We have selected two complementary bimetallic systems, Pd-Au and Pd-Ir. For bulk thermodynamics, Pd-Au is a random alloy, while Pd-Ir exhibits a miscibility gap. Whereas Pd-Au nanoparticles have been extensively studied for catalysis applications, almost nothing has been published on Pd-Ir catalysts. However, the combination of hydrogenation (Pd) and hydrogenolysis (Ir) metals is potentially interesting. The bimetallic particles will be supported on TiO2 and MgO. These crystalline oxides are well suited for characterization and exhibit distinct interactions with metals. The supported catalysts will be synthesized by pulsed laser deposition and colloidal methods.
The central characterization technique will be last-generation aberration-corrected transmission electron microscopy (TEM) and environmental TEM (ETEM). Several related methods will aim at determining the atomic structure, chemical structure (type of chemical arrangement: random, ordered, core-shell, segregated, etc.) and 3D morphology of the nanoparticles with ultrahigh resolution (UHRTEM). Electron microscopy will be completed by advanced global characterization techniques (in situ XAS, in situ XRD, XPS-ISS and CO-FTIR).
In order to establish correlations between structure and catalytic properties of the supported nanoalloys, a series of simple test-reactions will be assessed: selective hydrogenation of 1,3-butadiene (BSH), CO oxidation, selective ring opening of methylcyclohexane (MCHO).
The catalysts will be, in specific cases, subjected to thermal and/or reactive treatments prior to characterization. Our goal is to gain mechanistic insight into catalytic processes caused by the reaction conditions and causing catalyst modification in terms of structure and catalytic activity: particle restructuring (shape change, chemical segregation, etc.), sintering and coating of actives sites with atoms of the support (SMSI effect).

This interdisciplinary project (chemistry and physics) involves 3 academic partners (MPQ, IRCELYON, CINaM) and would last 36 months.